The present disclosure relates to a semiconductor laser device having a high light output at which thermal saturation occurs, and a low operating current even in a high-temperature and high-output operating state.
Semiconductor laser devices (hereinafter referred to also as semiconductor lasers) are widely used in various fields. For example, an AlGaInP semiconductor laser capable of providing red laser light in the 650 nm wavelength band is widely used as a light source in the field of an optical disk system represented by DVD. As a semiconductor laser, there has been known a laser which uses a dielectric film for a current block layer to reduce the number of times that crystal growth is performed in the process of producing the semiconductor laser, and thereby reduce fabrication cost and lead time (see, e.g., Japanese Laid-Open Patent Publication No. 2005-64328).
FIG. 21 shows an example of an AlGaInP semiconductor laser having such a structure.
FIG. 21 is a cross-sectional schematic diagram of the AlGaInP laser in a first conventional embodiment.
The semiconductor laser shown in FIG. 21 has a ridge stripe structure in which an n-type GaAs buffer layer 102, an n-type AlGaInP clad layer 103, a non-doped AlGaInP optical guide layer 104, a multiple quantum well active layer 105, a non-doped AlGaInP optical guide layer 106, a p-type AlGaInP clad layer 107, a p-type GaInP hetero buffer layer 108, a p-type GaAs cap layer 109, and a laminated current block layer 112 are formed on an n-type GaAs substrate 101, and an n-side electrode 113 and a p-side electrode 114 are further formed on the back surface of the n-type substrate 101 and on the contact layer 109, respectively.
The n-type GaAs substrate 101 is made of a Si-doped n-type GaAs substrate. The n-type GaAs buffer layer 102 is made of a Si-doped n-type GaAs layer (with a Si concentration n of 2×1018 cm−3 and a film thickness t of 0.5 μm). The n-type AlGaInP clad layer 103 is made of a Si-doped n-type (Al0.7Ga0.3)0.5In0.5P layer (with a carrier concentration of 1×1018 cm−3 and a film thickness t of 1.5 μm). The non-doped AlGaInP optical guide layer 104 is made of a non-doped (Al0.5Ga0.5)0.5In0.5P layer (with a film thickness t of 25 nm). The multiple quantum well active layer 105 is made of a non-doped (Al0.5Ga0.5)0.5In0.5P well (including three layers with a film thickness t of 5 nm), and a non-doped Ga0.5In0.5P well (including four layers with a film thickness t of 6 nm). The non-doped AlGaInP optical guide layer 106 is made of a non-doped (Al0.5Ga0.5)0.5In0.5P layer (with a film thickness t of 25 nm). The p-type AlGaInP clad layer 107 is made of a Zn-doped p-type (Al0.7Ga0.3)0.5In0.5P layer (with a carrier concentration of 1×1018 cm−3 and a film thickness t of 1.3 μm). The p-type GaInP hetero buffer layer 108 is made of a Zn-doped p-type Ga0.5In0.5P layer (with a carrier concentration of 1×1018 cm−3 and a film thickness t of 50 nm). The p-type GaAs cap layer 109 is made of a Zn-doped GaAs layer (with a carrier concentration of 1×1019 cm−3 and a film thickness t of 200 nm).
The current block layer 112 is made of a silicon nitride film 110 (with a film thickness t of 10 nm) and a hydrogenated amorphous silicon film 111 (with a film thickness t of 100 nm).
Additionally, in order to reduce damage to a ridge portion in the case of performing assembly by junction-down mounting, a pair of protruding portions 115 which are higher in level than the ridge portion are formed with the ridge portion being interposed therebetween.
In the semiconductor laser shown in FIG. 21, a current injected from the p-type GaAs cap layer 109 is constricted only to the ridge portion by the current block layer 112 made of a dielectric material, and concentratively injected into the multiple quantum well active layer 105 in the vicinity of the ridge bottom portion. In this manner, despite a small amount of injected current on the order of several tens of milliamperes, an inverted population state of carriers necessary for laser oscillation is implemented. At this time, light is generated through carrier recombination, and confined in a direction perpendicular to the multiple quantum well active layer 105 by both of the n-type AlGaInP clad layer 103 and the p-type AlGaInP first clad layer 107. In a direction parallel with the multiple quantum well active layer 105, the generated light can be confined because the current block layer 112 made of the dielectric material is lower in refractivity than the p-type AlGaInP first clad layer 107, and an effective refractivity inside the ridge can accordingly be set higher than that outside the ridge. As a result, laser oscillation can be caused when a gain resulting from the injected current surpasses a loss in a waveguide.
The dielectric film is deposited on each of the sidewall portions of the ridge by a thickness according to a deposition time thereof, and a deposition speed is substantially constant regardless of surface orientation. Therefore, when the dielectric film is used for a current block layer in a ridge-type laser, the dielectric film is deposited in a shape reflecting the shape of each of depressed portions outside the ridge so that, in regions outside the ridge, trench portions are formed between the ridge and a pair of the protruding portions (the regions with the protruding portions will be hereinafter referred to as wing regions).
With regard to the shapes of the trenches, such configurations as those shown in FIGS. 22 to 24 shown in Japanese Laid-Open Patent Publications Nos. 2003-158344 and HEI 10-173277 have been reported. That is, there have been proposed the configuration in which a ridge 121 has a tapered shape in the longitudinal direction of a resonator, while wing regions 120 are each formed in a direction parallel with a normal to a resonator surface (FIG. 22), the configuration in which the ridge 121 has a tapered shape in the longitudinal direction of the resonator, while the wing regions 120 are each formed parallel with the ridge 121 having the tapered shape (FIG. 23), and the configuration in which the ridge 121 has a uniform shape in the longitudinal direction of the resonator, while the wing regions 120 are each formed in a direction inclined with respect to the normal to the resonator surface (FIG. 24). Trenches 122 are trenches between the ridge 121 and the wing regions 120.